1. Field of the Invention
The present invention relates to an optical information recording medium and a method for manufacturing this medium, and more particularly relates to an optical information recording medium in which a light transmitting layer is disposed on one side of a textured substrate, and a warpage adjusting layer is disposed on the other side.
2. Description of the Related Art
Research into various kinds of optical information recording has been under way for some time in the field of information recording. Optical information recording makes it possible to raise the density, and also allows non-contact recording and reproduction, and is therefore being put to use in a wide range of applications as an inexpensive way to accomplish these goals. Blu-ray discs (BDs) are optical disks that allow higher-density recording than CDs or DVDs. These Blu-ray discs come as single-layer disks having a structure in which an information recording layer is provided on a transparent resin substrate with a thickness of 1.1 mm, and this layer is protected by a light transmitting layer, and as multilayer disks having a structure in which an intermediate layer is provided between a plurality of information recording layers, and this layer is protected by a light transmitting layer, for example.
However, when a light transmitting layer or a bar-code layer or other such layer of another material is laminated over a transparent resin substrate, the structure ends up being asymmetrical in the thickness direction. Consequently, changes in the environment, such as the temperature or humidity, can warp an optical disk.
In view of this, an optical disk was proposed in the past with which warpage was reduced by using a resin substrate with a groove formed on one side, forming a first UV curable resin film on the side of the substrate on which the groove is formed, and forming, on the other side, a second UV curable resin film that is flat and has a different thickness from that of the first UV curable resin film (see Japanese Laid-Open Patent Application H03-248341, for example).
Nevertheless, as the density of optical disks has risen, less and less warpage is permissible in optical disks because of its relationship to the focal position or the numerical aperture of the optical head. For example, warpage caused by changes in the environment, such as the temperature or humidity, exacerbates such problems as diminished reproduction of an optical disk and recording and reproduction signals, diminished tracking servo performance, and so forth. The inside of the recording and reproduction device (drive) of an optical disk often becomes quite hot, with the temperature rising to about 40 to 50° C., and therefore research has been conducted with the aim of reducing warpage over a range from temperatures near room temperature up to relatively high temperatures of about 100° C.
Meanwhile, even though it has been confirmed that warpage can be effectively prevented over a range from near room temperature up to high temperatures, it was unexpectedly found that warpage can still be very pronounced in the negative direction (the direction of the light projection plane) at temperatures below 0° C.
Usually, when an optical disk is inserted into a drive, the optical pickup used for reproduction learns the focus position according to the warpage of the disk, but if there is a large amount of warpage, the learning function may not be actuated because of the possibility of collision with the disk, which creates problems in the recording and reproduction of information.
Also, there is increasing demand for cost advantages, and there is a need for the development of an optical disk that has a simple structure while still having improved characteristics and avoiding such problems as warpage, and with which raw material and manufacturing costs are kept to a minimum, and a decrease in yield can be prevented by employing a simple manufacturing process.